1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine. For example, the air-fuel ratio control apparatus is mounted in a vehicle. In particular, the present invention relates to a technique for maintaining purification ratio of NOx at a high level even if a three-way catalyst is deteriorated.
2. Description of the Related Art
A conventional air-fuel ratio control apparatus includes: a three-way catalyst (hereinafter, simply referred to as “catalyst”) provided in an exhaust passage of an internal combustion engine for purifying HC, CO, and NOx in an exhaust gas at the same time; a first air-fuel ratio sensor for detecting a first air-fuel ratio at a position upstream of the catalyst; a second air-fuel ratio sensor for detecting a second air-fuel ratio at a position downstream of the catalyst; and a controller for controlling the air-fuel ratio. An oxygen storage amount of the catalyst is calculated based on the first air-fuel ratio and an intake air amount for controlling the air-fuel ratio for the internal combustion engine such that the oxygen storage amount matches a target oxygen storage amount (for example, see JP 2001-234789 A).
Operation of the conventional apparatus will be described.
As well known in the art, purification performance of the catalyst is high near the stoichiometric air-fuel ratio. If an operating point deviates from the stoichiometric air-fuel ratio, its purification efficiency lowers. Therefore, in order to address a problem due to temporal deviation of the air-fuel ratio, the catalyst has an oxygen storage capacity.
With the oxygen storage capacity, the catalyst takes in the oxygen in the exhaust gas when the operation is carried out on a lean side of the stoichiometric air-fuel ratio, so the catalytic atmosphere can be maintained at the stoichiometric air-fuel ratio until the oxygen storage amount is saturated.
The catalyst releases the oxygen when the operation is carried out on a rich side of the stoichiometric air-fuel ratio, so the catalytic atmosphere can be maintained at the stoichiometric air-fuel ratio.
The controller calculates the amount of oxygen absorbed into or released from the catalyst by integration based on an excess oxygen rate determined by conversion from the first air-fuel ratio and the intake air amount at this point. The controller controls the oxygen storage amount to a target oxygen storage amount for maintaining the catalytic atmosphere at the stoichiometric air-fuel ratio.
Further, since the first air-fuel ratio sensor is exposed in high temperature exhaust air, fluctuation occurs in outputs of the detection signal. In order to correct the fluctuation, the controller corrects the deviation from the stoichiometric air-fuel ratio using the second air-fuel ratio sensor, and keeps the air-fuel ratio at a position downstream of the catalyst at the stoichiometric air-fuel ratio.
Next, description will be made of purification characteristics for HC and NOx when the catalyst is in a new condition, and in a deteriorated condition.
For example, when the catalyst is in the new condition, and when the air-fuel ratio detected by the second air-fuel ratio sensor is near the stoichiometric air-fuel ratio, the HC purification rate is a maximum state. The HC purification ratio decreases as the air-fuel ratio further deviates from the stoichiometric air-fuel ratio toward the rich side or the lean side. However, the margin of the decreased purification ratio is small, and the HC purification rate has substantially flat characteristics.
On the other hand, when the air-fuel ratio is near the stoichiometric air-fuel ratio, the NOx purification rate is the maximum. The NOx purification rate reduces gradually on the rich side, and reduces sharply on the lean side.
As the deterioration of the catalyst progresses, the HC purification rate lowers in comparison with the HC purification rate when the catalyst is in the new condition. However, the purification rate is high when the air-fuel ratio is near the stoichiometric air-fuel ratio.
On the other hand, the NOx purification rate reduces significantly when the air-fuel ratio is not near a predetermined value (on the rich side of the stoichiometric air-fuel ratio). Even if the air-fuel ratio is near the stoichiometric air-fuel ratio, the purification rate reduces significantly.
In consideration of the NOx purification characteristics in correspondence with the deterioration level of the catalyst, as the deterioration level increases, the purification rate at the air-fuel ratio deviated from the predetermined value reduces much more. Even if the air-fuel ratio is near the stoichiometric air-fuel ratio, the purification rate reduces significantly.
As a result, when the air-fuel ratio is at the stoichiometric air-fuel ratio, if the catalyst is in the new condition, the nearly maximum NOx purification ratio is maintained. However, as the deterioration of the catalyst progresses, the NOx purification rate reduces significantly.
That is, when the catalyst is in the new condition, the purification rates of HC, CO, and NOx are designed to be high as long as the air-fuel ratio is near the stoichiometric air-fuel ratio. However, in the actual condition in use, the purification performance of the catalyst lowers owing to the deterioration of the catalyst due to various factors.
For example, hot exhaust air is a thermal deterioration factor. Since the particle structure of noble metals such as platinum, palladium, and rhodium in the catalyst deforms gradually, the purification performance of the noble metals lowers.
Further, components in the fuel such as lead, sulfur, and phosphor are poisonous deterioration factors. Those components are attracted to the noble metals, and the noble metals are poisoned. Therefore, the purification performance of the noble metals lowers.
That is, in the conventional apparatus, since the air-fuel ratio at a position downstream of the catalyst is controlled at the stoichiometric air-fuel ratio, the purification rate is kept at a high level when the catalyst is in the new condition. However, when the deterioration of the catalyst progresses, it is not possible to maintain the initial purification performance.
In the conventional air-fuel ratio control apparatus for an internal combustion engine, since the air-flow ratio at a position downstream of the catalyst is controlled at the stoichiometric air-fuel ratio, when the catalyst is in the new condition, the high purification ratio is maintained for each of HC and NOx. However, if the catalyst is in the deteriorated condition, the high purification rate is maintained for HC, but the NOx purification ratio reduces significantly.